1. Field of the Invention
The present invention relates to an apparatus for forming metal film and a process for forming metal films. More particularly, it relates to an apparatus for forming metal films, and a process for forming metal film, that can form a metal films at a high throughput.
2. Related Background Art
In electronic devices or integrated circuits making use of semiconductors, aluminum (Al), Al--Si or the like have been hitherto used in electrodes or wiring. In such use, Al is inexpensive, has a high electrical conductivity and a dense oxide film can be formed on its surface. Hence it has many advantages such that it is chemically protected and therefore stable and it can achieve good adhesion to Si.
With an increase in the degree of integration of integrated circuits such as LSIs, it has become particularly necessary in recent years to make wiring finer or form wiring in multi-layers. Hence, a new severe demand is directed to conventional Al wiring. As the size is made finer with an increase in the degree of integration, the surfaces of LSIs or the like tend to be formed in intense irregularity after going through the steps of oxidation, diffusion, thin-film deposition, etching and so forth. Electrodes or wiring metals, however, must be deposited on stepped surfaces without disconnecting or must be deposited in via holes that are minute in diameter and depth. In the case of 4 Mbit or 16 Mbit DRAMs (dynamic RAM), the aspect ratio (via hole depth/via hole diameter) of via holes in which metals such as Al must be deposited is 1.0 or more and the via hole diameter itself is 1 .mu.m or less. It is accordingly required to provide a technique that enables good deposition of Al even in the via holes having a large aspect ratio.
Al must be deposited not only to fill up the via holes but also to form wiring on insulating films and the resulting deposited films must be a good quality.
For the formation of such deposited films, conventional sputtering can not meet the demands stated above.
In conventional sputtering, films are formed in an extremely small thickness at relatively stepped portions or sidewalls of insulating film to cause disconnection in an extreme case, resulting in greatly lower reliability of LSIs. As for bias sputtering, the process has technical problems which are difficult to solve, e.g., damage by charged particles or a limit in deposition rate.
Some techniques are proposed which intend to solve such technical problems.
One of them is in a U.S.A. patent application filed on Sep. 24, 1990 under Ser. No. 587,045, entitled "Process for Forming Deposited Film", and is disclosed in European Patent Publication No. 420,595. This process comprises Selectively depositing Al in a via hole, the bottom of which is formed of an electron donative surface, thereafter generating plasma in the vicinity of the surface of a substrate to modify the electron non-donative surface formed of, e.g., SiO.sub.2 or Si.sub.3 N.sub.4, and depositing Al on the modified surface.
Here, the modification is meant to bring the surface into the state that free electrons are seemingly present even if the surface is electron non-donative, and can participate in the surface reaction. According to plasma surface modification treatment, it is possible to cut off Si--O bonds to generate electrons on account of incomplete bonds.
Such electrons accelerate chemical reaction as a result of electron transfer between the substrate and starting material gas molecules adsorbed on the substrate surface.
FIG. 1 illustrates an apparatus for such conventional metal film formation. Reference numeral 1 denotes a semiconductor wafer on which the metal film is to be formed, and which is placed on a substrate holder 3 provided with a heating means 4.
To a reaction chamber 2, an exhaust system is provided via a gate valve 7 and a slow-leak valve 8 so that the inside of the reaction chamber can be evacuated. Above the reaction chamber 2, a bubbler 6 and a mixing device 5 are provided as a starting material gas feeding means that feeds starting material gas into the reaction chamber. Reference numeral 10 denotes a transport chamber through which the wafer 1 is carried in or taken out of the reaction chamber 2, which is provided via a valve 13. The inside of the chamber 10 is connected to an exhaust system 12 via a valve 11 so that the inside of the chamber 12 can be independently evacuated.
Reference numeral 16 denotes an electrode that generates plasma; and 14, a power source that supplies an electrical energy for generating the plasma, which is grounded at its terminal end.
In the process for forming metal film making use of the apparatus for forming metal film described above, electrons must be supplied uniformly over the whole deposition surface of the substrate, and hence this is a method of processing one sheet of substrate (wafer) in a one time deposition process. Thus, it has been difficult to obtain a throughput high enough to achieve mass production.
Now, the present inventors have tried a method of forming metal films using the FIG. 1 apparatus whose reaction chamber has been enlarged so that a plurality of substrates can be held.
However, compared with the FIG. 1 apparatus, a poor uniformity tended to result with regard to the metal films on different wafers and the metal film on the same wafer which were formed by non-selective deposition utilizing plasma, and it was difficult to always obtain uniform metal films.